A highly efficient procedure for transferring functional genes into mammalian cells has been developed using retroviral vectors as a delivery system. When mouse bone marrow cells are infected in vitro with a Neo-R gene and reinjected into a lethally irradiated recipient mouse, 85%-90% of the stem cells (CFU-S) can be shown to carry an intact copy of the Neo-R gene. The majority of these cells produce the Neo-R gene product: phosphotransferase. Retroviral vectors containing the human gene for the enzyme adenosine deaminase (ADA) as well as the Neo-R gene have been made. Using the knowledge obtained from the murine system, a non-human primate autologous bone marrow transplantation/gene transfer protocol has been developed. Low levels of the human ADA gene have been expressed in monkey peripheral blood cells. These studies are preliminary to attempting human gene therapy in patients suffering from ADA severe combined immunodeficiency disease.